Designers, manufacturers, and users of electronic computers and computing systems require reliable and efficient equipment for storage and retrieval of information in digital form. Conventional storage systems, such as magnetic disk drives, are typically utilized for this purpose and are well known in the art. However, the amount of information that is digitally stored continually increases, and designers and manufacturers of magnetic recording media work to increase the storage capacity of magnetic disks.
In conventional magnetic disk data/information storage, the data/information is stored in a continuous magnetic thin film overlying a substantially rigid, non-magnetic disk. Each bit of data/information is stored by magnetizing a small area of the thin magnetic film using a magnetic transducer (write head) that provides a sufficiently strong magnetic field to effect a selected alignment of the small area (magnetic grain) of the film. The magnetic moment, area, and location of the small area comprise a bit of binary information which must be precisely defined in order to allow a magnetic read head to retrieve the stored data/information.
Such conventional magnetic disk storage media incur several drawbacks and disadvantages which adversely affect realization of high areal density data/information storage, as follows:
(1) the boundaries between adjacent pairs of bits tend to be ragged in continuous magnetic films, resulting in noise generation during reading; and
(2) the requirement for increased areal recording density has necessitated a corresponding decrease in recording bit size or area. Consequently, recording bit sizes of continuous film media have become extremely minute, e.g., on the order of nanometers (nm). In order to obtain a sufficient output signal from such minute bits, the saturation magnetization (Ms) and thickness of the film must be as large as possible. However, the magnetization quantity of such minute bits is extremely small, resulting in a loss of stored information due to magnetization reversal by “thermal fluctuation”, also known as the “superparamagnetic effect”.
Regarding item (2) above, it is further noted that for longitudinal type continuous magnetic media, wherein the magnetic easy axis is oriented parallel to the film plane (i.e., surface), magnetization reversal by the superparamagnetic effect may occur even with relatively large magnetic particles or grains, thereby limiting future increases in areal recording density to levels necessitated by current and projected computer-related applications. On the other hand, for perpendicular type continuous magnetic media, wherein the magnetic easy axis is oriented perpendicular to the film plane (i.e., surface), growth of the magnetic particles or grains in the film thickness direction increases the volume of magnetization of the particles or grains while maintaining a small cross-sectional area (as measured in the film plane). As a consequence, onset of the superparamagnetic effect can be suppressed for very small particles or grains of minute width. However, further decrease in grain width in perpendicular media necessitated by increasing requirements for areal recording density will inevitably result in onset of the superparamagnetic effect even for such type media.
The superparamagnetic effect is a major limiting factor in increasing the areal recording density of continuous film magnetic recording media. Superparamagnetism results from thermal excitations which perturb the magnetization of grains in a ferromagnetic material, resulting in unstable magnetization. As the grain size of magnetic media is reduced to achieve higher areal recording density, the superparamagnetic instabilities become more problematic. The superparamagnetic effect is most evident when the grain volume V is sufficiently small such that the inequality KμV/kBT>40 cannot be maintained, where Kμ is the magnetic crystalline anisotropy energy density of the material, kB is Boltzmann's constant, and T is the absolute temperature. When this inequality is not satisfied, thermal energy demagnetizes the individual magnetic grains and the stored data bits are no longer stable. Consequently, as the magnetic grain size is decreased in order to increase the areal recording density, a threshold is reached for a given Kμ and temperature T such that stable data storage is no longer possible.
So-called “patterned” or “bit patterned” magnetic media (“BPM”) have been proposed as a means for overcoming the above-described problem of conventional continuous magnetic media associated with magnetization reversal via the superparamagnetic effect, e.g., as disclosed in U.S. Pat. No. 5,956,216, the entire disclosure of which is incorporated herein by reference. The term “bit patterned media” (“BPM”) generally refers to magnetic data/information storage and retrieval media wherein a plurality of discrete, independent regions of magnetic material which form discrete, independent magnetic elements that function as recording bits are formed on a non-magnetic substrate. Since the regions of ferromagnetic material comprising the magnetic bits or elements are independent of each other, mutual interference between neighboring bits can be minimized. As a consequence, bit patterned magnetic media are advantageous vis-à-vis continuous magnetic media in reducing recording losses and noise arising from neighboring magnetic bits. In addition, patterning of the magnetic layer advantageously increases resistance to domain wall movement, i.e., enhances domain wall pinning, resulting in improved magnetic performance characteristics.
Generally, each magnetic bit or element has the same size and shape, and is composed of the same magnetic material as the other elements. The elements are arranged in a regular pattern over the substrate surface, with each element having a small size and desired magnetic anisotropy, so that, in the absence of an externally applied magnetic field, the magnetic moments of each discrete magnetic element will be aligned along the same magnetic easy axis. The magnetic moment of each discrete magnetic element therefore has only two states: the same in magnitude but aligned in opposite directions. Each discrete magnetic element forms a single magnetic domain or bit and the size, area, and location of each domain is determined during the fabrication process.
During writing operation of patterned media, the direction of the magnetic moment of the single magnetic domain element or bit is flipped along the easy axis, and during reading operation, the direction of the magnetic moment of the single magnetic domain element or bit is sensed. While the direction of the magnetic easy axis of each of the magnetic domains, elements, or bits can be parallel or perpendicular to the surface of the domain, element, or bit, corresponding to conventional continuous longitudinal and perpendicular media, respectively, bit patterned media comprised of domains, elements, or bits with perpendicularly oriented magnetic easy axis are advantageous in achieving higher areal recording densities for the reasons given above.
Bit patterned media in disk form offer a number of advantages relative to conventional disk media. In principle, the writing process is greatly simplified, resulting in much lower noise and lower error rate, thereby allowing much higher areal recording density. In bit patterned media, the writing process does not define the location, shape, and magnetization value of a bit, but merely flips the magnetization orientation of a patterned single domain magnetic structure. Also in principle, writing of data can be essentially perfect, even when the transducer head deviates slightly from the intended bit location and partially overlaps neighboring bits, as long as only the magnetization direction of the intended bit is flipped. By contrast, in conventional magnetic disk media, the writing process must define the location, shape, and magnetization of a bit. Therefore, with such conventional disk media, if the transducer head deviates from the intended location, the head will write to part of the intended bit and to part of the neighboring bits. Another advantage of bit patterned media is that crosstalk between neighboring bits is reduced relative to conventional media, whereby areal recording density is increased. Each individual magnetic element, domain, or bit of a patterned medium can be tracked individually, and reading is less jittery than in conventional disks.
As utilized herein, the general expression “patterned recording media” is taken as encompassing different types of pattern formation and different types of recording media with patterned surfaces, including, but not limited to, servo-patterned magnetic and magneto-optical (“MO”) media, track-patterned (i.e., discrete track) magnetic media, bit patterned magnetic (“BPM”) media, patterned read-only (“ROM”) media, and wobble-groove patterned readable compact disk (“CD-R”), readable-writable compact disk (“CD-RW”) media, and digital video disk (“DVD”) media. Such media have been fabricated by a variety of processing techniques, including etching processing such as reactive ion etching, sputter etching, ion milling, and ion irradiation to form a pattern comprising magnetic and non-magnetic surface areas in a layer of magnetic material on a media substrate. Several of the these processing techniques have relied upon selective removal of portions of the layer of magnetic material to form the pattern of magnetic and non-magnetic surface areas; whereas others of the processing techniques have relied upon partial removal of selected areas of the media substrate on which the magnetic layer is formed, thereby resulting in different transducer head/media surface spacings having an effect similar to formation of a pattern of magnetic and non-magnetic surface areas in the layer of magnetic material. However, a drawback associated with each of these techniques is formation of topographical patterns in the surface of the media, engendering media performance concerns such as transducer head flyability and corrosion, e.g., due to uneven lubricant thickness and adhesion.
A recently developed low cost alternative technique for fine dimension pattern/feature formation (i.e., sub-100 nm structures/features) in a substrate surface is thermally assisted nano-imprint lithography, as for example, described in U.S. Pat. Nos. 4,731,155; 5,772,905; 5,817,242; 6,117,344; 6,165,911; 6,168,845 B1; 6,190,929 B1; and 6,228,294 B1, the entire disclosures of which are incorporated herein by reference. A typical thermally assisted nano-imprint lithographic process for forming nano-dimensioned patterns/features in a substrate surface is illustrated with reference to the schematic, cross-sectional views of FIGS. 1 (A)-1 (D).
Referring to FIG. 1 (A), shown therein is a stamper/imprinter 10 (also referred to in the related art as a “mold” or “template”) including a main (or support) body 12 having upper and lower opposed surfaces, with an imprinting layer 14 formed on the lower opposed surface. As illustrated, stamper/imprinter 14 includes a plurality of features 16 having a desired shape or surface contour. A workpiece 18 carrying a thin film layer 20 on an upper surface thereof is positioned below, and in facing relation to the molding layer 14. Thin film layer 20, of a thermoplastic polymer material, e.g., polymethylmethacrylate (PMMA), may be formed on the substrate/workpiece surface by any appropriate technique, e.g., spin coating.
Adverting to FIG. 1 (B), shown therein is a compressive molding step, wherein stamper/imprinter 10 is pressed into the thin film layer 20 in the direction shown by arrow 22, so as to form depressed, i.e., compressed, regions 24. In the illustrated embodiment, features 16 of the imprinting layer 14 are not pressed all of the way into the thin film layer 20 and thus do not contact the surface of the underlying substrate 18. However, the top surface portions 24a of thin film 20 may contact depressed surface portions 16a of imprinting layer 14. As a consequence, the top surface portions 24a substantially conform to the shape of the depressed surface portions 16a, for example, flat. When contact between the depressed surface portions 16a of imprinting layer 14 and thin film layer 20 occurs, further movement of the imprinting layer 14 into the thin film layer 20 stops, due to the sudden increase in contact area, leading to a decrease in compressive pressure when the compressive force is constant.
FIG. 1 (C) shows the cross-sectional surface contour of the thin film layer 20 following removal of stamper/imprinter 10. The imprinted thin film layer 20 includes a plurality of recesses formed at compressed regions 24 which generally conform to the shape or surface contour of features 16 of the molding layer 14. Referring to FIG. 1 (D), in a next step, the surface-imprinted workpiece is subjected to processing to remove the compressed portions 24 of thin film 20 to selectively expose portions 28 of the underlying substrate 18 separated by raised features 26. Selective removal of the compressed portions 24 may be accomplished by any appropriate process, e.g., reactive ion etching (RIE) or wet chemical etching.
The above-described imprint lithographic processing is capable of providing sub-micron-dimensioned features, as by utilizing a stamper/imprinter 10 provided with patterned features 16 comprising pillars, holes, trenches, etc., by means of e-beam lithography, RIE, or other appropriate patterning method. Typical depths of features 16 range from about 5 to about 200 nm, depending upon the desired lateral dimension. The material of the imprinting layer 14 is typically selected to be hard relative to the thin film layer 20, the latter comprising a thermoplastic material which is softened when heated. Thus, materials which have been proposed for use as the imprinting layer 14 include metals, dielectrics, semiconductors, ceramics, and composite materials. Suitable materials for use as thin film layer 20 include thermoplastic polymers which can be heated to above their glass temperature, Tg, such that the material exhibits low viscosity and enhanced flow.
Referring to FIGS. 2 (A)-2 (D), shown therein, in simplified, schematic cross-sectional views, is a series of process steps for illustrating fabrication of bit patterned or servo patterned magnetic recording media utilizing thermal imprint lithography as part of the processing methodology.
In FIG. 2 (A), a layer 70 of a thermoplastic polymer material, e.g., PMMA, covers a media substrate 72, e.g., of a suitable material (which substrate may comprise at least a surface layer of a magnetically soft material when the resultant medium is a perpendicular medium). Opposite the polymer layer 70 is a stamper/imprinter (sometimes referred to as a “mold”) 74 which includes a patterned plurality of downwardly extending features 76, e.g., pillars as in the illustrated embodiment, of preselected dimensions and arrangement for forming a desired pattern in the polymer layer 70, e.g., a servo pattern or a discrete bit pattern. As indicated by the downwardly facing arrows in FIG. 2 (A), the stamper/imprinter 74 is moved toward the polymer layer 70 to form an imprinted pattern therein which is a negative image of the pattern of the downwardly extending features 76 in the form of recesses 78, as shown in FIG. 2 (B). During the imprinting process, the thermoplastic polymer layer 70 is typically maintained at an elevated temperature which facilitates the imprinting, i.e., at a temperature close to the melting or glass transition temperature Tg of the polymer material. As in the embodiment shown in FIG. 1, the imprinted polymer layer may, if desired, be subjected to further processing to effect complete removal of the bottom portions of the recesses 78 to thereby expose the surface of substrate 72. Recesses 78 are then filled with a layer 80 of a magnetic recording material (or a plurality of stacked layers including seed, intermediate, etc., layers in addition to a layer of magnetic recording material), as shown in FIG. 2 (C). Excess material of layer 80 overfilling the recesses 78 (as seen in FIG. 2 (C)) is then removed via a planarization process, e.g., chemical-mechanical polishing (CMP), to leave a plurality of single elements or bits 82 each forming a single magnetic domain of a bit patterned medium.
Stampers/imprinters suitable for use in performing the foregoing patterning processes have conventionally been made from a number of materials such as etched Si wafers, etched quartz or glass, and electroformed metals, e.g., electroformed Ni, and may be manufactured by a sequence of steps as schematically illustrated in FIG. 3, which steps include providing a “master” comprised of a substantially rigid substrate with a patterned layer of a resist material thereon. The pattern, which is formed in the resist layer by conventional lithographic techniques, including, e.g., e-beam or laser beam exposure of selected areas of the resist, comprises a plurality of projections and depressions corresponding (in positive or negative image form, as necessary) to the desired pattern, e.g., a servo pattern, to be formed in the surface of the stamper/imprinter. According to the process shown in FIG. 3, stampers/imprinters are made from the “master” by initially forming a thin, conformal layer of an electrically conductive material (e.g., Ni) over the patterned resist layer and then electroforming a substantially thicker (“blanket”) metal layer (e.g., Ni) on the thin layer of electrically conductive material, which electroformed blanket layer replicates the surface topography of the resist layer. Upon completion of the electroforming process, the stamper/imprinter is separated from the “master”.
In practice, however, since the “master” with fragile resist layer thereon is effectively destroyed upon separation of the stamper/imprinter from the “master”, a process has been developed involving forming a “family” of stampers/imprinters, as schematically illustrated in FIG. 4. As shown in the figure, the stamper/imprinter formed directly from the “master” is termed a “father” and has a reverse (i.e., negative) replica of the topographical pattern of the “master”. The “father” is then utilized for forming several (illustratively two) “mothers” therefrom (e.g., as by a process comprising electroforming, as described above), and each “mother” is in turn utilized for forming several (illustratively two, for a total of four) “sons” therefrom (also by a process comprising electroforming). The “sons” are positive replicas of the “father” and are utilized as the stampers/imprinters for media patterning. Since, as described above, the “master” is effectively destroyed in the process of making the “father” therefrom, the “family” making process avoids the need for repeatedly manufacturing “master” stampers/imprinters by preserving the “father” and utilizing the “sons”. Therefore, process time and cost of making “masters” is substantially reduced by means of the “family” making process.
The thus-formed “sons” are then subjected to further processing for forming stampers/imprinters with a desired dimension (i.e., size) and geometrical shape or contour, e.g., an annular disk-shaped stamper/imprinter for use in patterning of annular disk-shaped media such as hard disks, which stampers/imprinters necessarily include a circularly-shaped central aperture defining an inner diameter (“ID”) and a circularly-shaped periphery defining an outer diameter (“OD”).
The “family” making process, as described supra, is made possible/practical only if the “mothers” are readily separated from the “father” without incurring damage to the patterned surface(s), and the “sons” are similarly readily separated from the “mothers” without incurring damage to the patterned surface(s). As a consequence, the patterned surfaces of the “father” and the “mothers” are each provided with a coating layer of a material, termed a “release” layer and typically comprised of a passivating material, prior to formation of the respective “mothers” and “sons”, for facilitating separation, i.e., “release”, of the “mothers” from the “father” and the “sons” from the “mothers”.
Fabrication of the stampers/imprinters is a key factor in the processing methodology for patterned media such as bit and servo patterned magnetic recording media. As indicated above, one process for fabricating stampers/imprinters for use in manufacturing patterned media comprises steps of: e-beam writing a desired pattern in a resist layer formed on a Si wafer substrate to form a “master”, electroplating/electroforming Ni thereon to form a Ni “father”, electroplating/electroforming Ni on the “father” to generate at least one “mother”, and electroplating/electroforming Ni on the at least one “mother” to generate at least one “son”. While the “family” making process for forming stampers/imprinters has resulted in great reduction in manufacturing costs, the use of Ni-based stampers/imprinters has encountered several problems, as follows: (1) the pattern features have very small dimensions with linear and irregularly contoured sidewalls, resulting in physical damage, e.g., breakage, to the pattern when separating the mothers from the fathers or when separating the sons from the mothers. Stated differently, pattern replication fidelity from one hard surface to another hard surface has reached a limit due to the extremely small feature sizes necessary for formation of certain types of patterned media, e.g., ultra-high areal recording density bit patterned media; (2) application of the necessary release layer to the Ni surfaces is very difficult, making it correspondingly difficult to achieve effective and durable imprinting; and (3) the difference (i.e., mismatch) in thermal expansion coefficient (“CTE”) between the Ni-based stampers/imprinters and the resist (thermoplastic polymer) and substrate materials further reduces replication fidelity.
Therefore, while nano-imprint lithographic techniques, such as described above, afford the possibility of a low-cost, mass manufacturing technology for fabrication of sub-100 nm structures, features, etc., for semiconductor ICs, integrated optical, magnetic, and mechanical devices, the problem of non-uniform replication and sticking of the thermoplastic polymer materials to the patterned imprinting surface when the latter is applied to a large-area substrate, e.g., as in the formation of patterns in 95 mm diameter disks used in hard disk drives, arising from differences in thermal expansion/contraction characteristics of the stamper/imprinter and substrate materials, has not been adequately or satisfactorily addressed.
For example, according to conventional practices in thermal imprint lithography, it is normal for the components of the imprinting system, i.e., substrate, resist layer, and stamper/imprinter to undergo large thermal swings or cycling, e.g., within a range of about 100° C. The 100° C. increase in temperature experienced by the thin film resist layer formed of a thermoplastic polymer causes the viscosity to decrease and hence increase the fluid flow characteristics thereof, which in turn, allows accurate replication of the features of the stamper/imprinter surface. However, a significant problem associated with this technique when utilized in certain applications is the dissimilar thermal expansion/contraction characteristics of the imprinting surface and thin film resist materials due to their entirely different properties, such as the CTE, which dissimilarity results in degradation of imprint quality, as by deformation of and/or damage to the replicated thin film resist layer after the imprinting process is completed.
As described above, stampers/imprinters have conventionally been fabricated by electroforming Ni (or Cu) onto a master plate comprising a patterned photoresist, or by etching through a substrate, e.g., of silicon (Si), coated with a layer of patterned photoresist. The former technique is typically utilized in the replication of vinyl audio records and optical disks; whereas the latter technique has been utilized to fabricate stampers/imprinters having very small feature sizes, e.g., ˜20 nm, by means of e-beam techniques. However, the thermal expansion/contraction characteristics of these materials are substantially and significantly different from those of either the glass, ceramic, glass-ceramic composite, or nickel-phosphorus coated aluminum (Al/Ni-P) substrates typically utilized in fabricating various types of recording media in disk form, e.g., magnetic media for use in hard disk drives, which differences in thermal expansion/contraction characteristics disadvantageously result in the above-mentioned degradation in imprint quality, e.g., resist deformation, poor mold release (sticking) causing resist peeling, and damage leading to loss of dimensional integrity, pattern/feature definition, etc.
AFM images and cross-sectional profiles of replicated features obtained by imprinting a substrate/workpiece comprised of a layer of PMMA on an Al/NiP substrate, utilizing a Ni-based stamper/imprinter according to the above-described process, typically indicate poor replication quality and partial peeling of the PMMA layer from the surface of the glass substrate is frequently observed upon separation of the Ni-based stamper/imprinter from the PMMA layer. It has been determined that the peeling was caused by relative movement between the stamper/imprinter and the substrate/workpiece which occurred during the temperature cycling of the process, since the thermal expansion/contraction characteristics of the Ni-based stamper/imprinter and the Al/NiP-based substrate/workpiece are significantly different. The frequency of tearing and lifting off of the thermoplastic polymeric resist layer from the media substrate during separation of the stamper/imprinter from the imprinted substrate/workpiece is increased when the thermal imprint process is applied to large area substrates/workpieces, e.g., 95 mm diameter disks utilized in the manufacture of hard disk media.
In view of the foregoing, there exists a need for improved stampers/imprinters which are free of the above-described problems, drawbacks, and disadvantages problems, drawbacks, and disadvantages attendant upon the use of Ni-based and similar type “father”, “mother”, and “son” stampers/imprinters in patterning of recording media. Moreover, there exists a need for methodologies which facilitate rapid, reliable, and cost-effective manufacture of the improved stampers/imprinters for use in rapid, reliable, accurate, and cost-effective patterning of a variety of types of recording media by means of thermally assisted nano-imprint lithography. The recording media types which may be fabricated according to the inventive means and methodology include, but are not limited to, ultra-high areal recording density bit patterned magnetic media, servo patterned magnetic and magneto-optical (MO) recording media, and various types of CD and DVD recording media.
The present invention addresses and solves the aforementioned problems, drawbacks, and disadvantages associated with the use of conventional stampers and manufacturing techniques therefor, while maintaining full compatibility with the requirements of cost-effective manufacturing technology.